The invention relates to a method and a device for the catalytic conversion of nitrogen oxides contained in the off-gass from a combustion system, in particular in an exhaust gas from an internal combustion engine operated with excess air, for example a diesel engine.
During the combustion of a fossil fuel, such as petroleum or coal, in a combustion system, in particular of diesel fuel in a diesel engine, nitrogen oxides that are hazardous to the environment are formed, inter alia. To reduce the emissions of nitrogen oxides to the environment, the use of a catalytic converter disposed in the off-gas line of a combustion system is known, inter alia, in the power plant sector. The catalytic converter is used to catalytically convert the nitrogen oxides contained in the off-gas into substances that are not hazardous.
In an internal combustion engine that is operated with excess air, nitrogen oxides are removed from the exhaust gas using, for example, a selective catalytic reduction (SCR) process. In this case, a reducing agent is introduced into the exhaust gas before it flows through a so-called SCR or DeNOx catalytic converter, which reducing agent converts the nitrogen oxides which are contained in the exhaust gas, in the presence of oxygen, at the catalytic converter to form harmless nitrogen and water. The reducing agent used is generally ammonia. The reducing agent is introduced into the exhaust gas in the form, for example, of a reducing agent solution from which the actual reducing agent is released. In the case of ammonia, a reducing agent solution of this type is, for example, an aqueous urea solution. In this respect, see the Siemens brochure, titled xe2x80x9cSINOx, Stickoxidminderung fxc3xcr stationxc3xa4re Dieselmotorenxe2x80x9d [SINOx, Nitrogen Oxide Abatement For Steady-State Diesel Engines], 1997, order No. A96001-U91-A232.
When reducing the levels of nitrogen oxides using the SCR process, it is always necessary for an amount of reducing agent that is adapted to the current nitrogen oxide emissions to be introduced into the exhaust gas. This on the one hand leads to a high conversion rate for the nitrogen oxides at the catalytic converter and on the other hand prevents too much reducing agent from being introduced, which then leaves the catalytic converter together with the exhaust gas and enters the environment. The emission of reducing agent into the environment is also known as slippage. This phenomenon is to be avoided in particular when using ammonia, in order to avoid additional pollution of the environment.
Particularly in the case of combustion systems that are not operated in a steady state, it is difficult to determine the amount of reducing agent that needs to be introduced per unit time. Examples of combustion systems which are operated in a non steady state include diesel engines which are used in the automotive sector and are operated with frequent load changes. Therefore, the emission of nitrogen oxides may vary considerably within short periods of time. Consequently, it is also necessary for the amount of reducing agent metered in to vary quickly and to be accurately adjusted. The amount of reducing agent to be introduced therefore needs to be controlled according to demand. The current demand is determined on the basis of parameters that characterize the operating state of the combustion system. In a diesel engine, these parameters are, for example, the engine speed, the torque, the operating temperature or the fuel consumption. It is known from Published, Non-Prosecuted German Patent Application DE 19 536 571 A1 to additionally use parameters which characterize the operating state of the catalytic converter. Examples of these parameters are the capacity of the catalytic converter to store the reducing agent, the operating temperature and/or the catalytic activity of the catalytic converter.
The amount of reducing agent required for conversion of the nitrogen oxides is determined from the various parameters, for example on the basis of a characteristic diagram. To determine the volume of reducing agent solution which is to be metered per unit time, when using a reducing agent solution it is additionally necessary to take into account the properties, for example the concentration, of this solution. Generally, the amount of reducing agent is determined in such a way that slightly less reducing agent than is required for conversion of the nitrogen oxides is fed to the catalytic converter, so that slippage is avoided under all circumstances. The catalytic converter is therefore operated at below the conversion rate that is theoretically possible. The conversion rate indicates the proportion of nitrogen oxides that are reduced at the catalytic converter.
It is accordingly an object of the invention to provide a method and a device for the catalytic reduction of nitrogen oxides contained in the off-gas from a combustion system that overcomes the above-mentioned disadvantages of the prior art methods and devices of this general type, in which slippage of the reducing agent is reliably avoided and, at the same time, a high conversion rate for the nitrogen oxides at the catalytic converter is achieved.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for a catalytic reduction of nitrogen oxides contained in a medium flow output from a combustion system. The method includes the steps of:
a) channeling an off-gas towards and through a catalytic converter;
b) metering a reducing agent solution into the off gas before it flows through the catalytic converter;
c) determining an amount of dissolved reducing agent to be metered per unit time according to demand; and
d) using a density of the reducing agent solution to determine a volume of the reducing agent solution to be metered per unit time.
According to the invention, to achieve the object in the method for the catalytic reduction of nitrogen oxides, in particular of nitrogen oxides contained in the exhaust gas from an internal combustion engine operated with excess air, the off gas flows through a catalytic converter. A reducing agent solution is metered into the off gas before it flows through the catalytic converter, and the amount of dissolved reducing agent which is to be metered per unit time is determined according to demand. The density of the reducing agent solution is used to determine the volume of reducing agent solution that is to be metered per unit time.
The invention is based on the consideration that, if the reducing agent solution is metered volumetrically, the amount of reducing agent metered in is influenced by the density of the reducing agent solution. Density fluctuations occur primarily in the event of temperature changes. As a result of the density being included, the inaccuracy of metering caused by density fluctuations is largely prevented and a high metering accuracy is achieved. As a result, the desired conversion rate is achieved with the maximum possible accuracy without slippage occurring.
The reducing agent solution used is preferably an aqueous urea solution. The urea solution is metered into the hot off gas. In the process, the actual reducing agent, namely ammonia, is released from the dissolved urea. Together with the nitrogen oxides, the ammonia enters the catalytic converter, where it reduces the nitrogen oxides to form nitrogen on the catalytically active surface.
To determine the density, it is preferable to measure the temperature of the reducing agent solution and to control the volume of reducing agent solution to be metered as a function of the density derived from the temperature. The temperature is the significant determining parameter for the density and is easy to determine using standard temperature sensors. Since the reducing agent solution is in liquid form and is virtually incompressible, the density is substantially unaffected by the influences of pressure.
The temperature measurement may be carried out as early as in a reservoir for the reducing agent solution or preferably immediately upstream of a metering device for the volumetric metering of the reducing agent solution. Determining the temperature immediately upstream of the metering device provides a higher level of accuracy, since the actual temperature of the reducing agent solution at the location of the metering device is determined. By contrast, if the temperature is measured in or at the reservoir, heat losses may under certain circumstances occur in a feed line leading to the metering device, and the heat losses should be taken into account when determining the density.
The volume to be metered is advantageously determined from a characteristic curve that shows the relationship between the temperature and the density of the reducing agent solution.
As a result, the volume to be metered is easy to read out from the characteristic curve and does not have to be calculated on an ongoing basis. It is therefore sufficient for the temperature/density relationship to be determined only on a one off basis, either experimentally or by calculation. It is advantageous for a plurality of characteristic curves for different reducing agent solutions to be recorded in a memory element. The reducing agent solutions may differ, for example, in terms of their concentration or their composition. The reducing agent used is generally urea, and the solvent used is generally water.
It is preferable for the temperature of the reducing agent solution to be controlled, so that its density is set at a fixed value, since the density is defined at a predetermined temperature. The advantage of direct temperature control of the reducing agent solution is that the density of the reducing agent solution does not have to be determined by a separate measurement, for example a temperature measurement.
In this case, the temperature of the reducing agent solution is controlled either in the reservoir for the reducing agent solution or immediately upstream of the metering device. Disposing a temperature-control device immediately upstream of the metering device offers the advantage that any heat losses which occur as a result of heat being dissipated by radiation between temperature control device and metering device are negligible.
In addition to determining the temperature of the reducing agent solution, controlling the temperature of the reducing agent solution offers an alternative or additional possibility for using the density of the reducing agent solution to determine the volume of the reducing agent solution which is to be metered per unit time. It is particularly advantageous for the two options to be combined with one another in order to achieve a high metering accuracy. The additional measurement of the temperature leads to a higher level of accuracy during the regulation of the density. This may be advantageous in particular during initial operation, when the temperature control device has not yet set the reducing agent solution to the predetermined temperature.
The temperature is preferably controlled by a temperature control device or heating configuration, specifically, by an NTC heater element. The NTC heater element is an electrical resistance heater element and is characterized in that its resistance has a negative temperature coefficient (NTC), i.e. its resistance and therefore its heating capacity decrease as the temperature rises. The NTC heater element is therefore almost self regulating, so that it can be used to achieve a desired temperature in the reducing agent solution in a particularly simple way without complex control of the heating configuration being required.
The NTC heater element is expediently used as a temperature sensor as well. For this purpose, the resistance of the NTC heater element is determined, representing an unambiguous function of the temperature. As a result, a single element can be used to simultaneously heat the reducing agent solution to a predetermined temperature and directly determine the instantaneous temperature of the reducing agent solution.
In a preferred configuration, the concentration of the reducing agent solution is used to determine the volume to be metered. The concentration of the reducing agent solution is generally an important determining parameter for the volume of reducing agent solution to be metered, since the concentration represents a measure of the amount of reducing agent that is actually dissolved.
Since the concentration may be subject to fluctuations, it is expedient for the concentration to be determined or regulated in addition to the density. The fluctuations in the concentration are caused, for example, by the effects of evaporation or production related differences between various reducing agent solutions which occur when the solution levels are topped up. The concentration is monitored, for example, by a measuring member or, as an alternative or in addition, is set to a predeterminable level by active regulation. For the active regulation, the reducing agent solution is, for example, thermostated to a predeterminable temperature. If the reducing agent solution is in the form of a saturated solution at the predetermined temperature, a fixedly defined equilibrium concentration in accordance with the phase diagram is established between the reducing agent and the solvent. Therefore, corresponding thermostating or temperature control of the reducing agent solution fixes both its concentration and its density in a particularly advantageous way, so that the volume to be metered can be determined very accurately.
With the foregoing and other objects in view there is further provided, in accordance with the invention, a device for catalytically reducing nitrogen oxides contained in an off-gas output from a combustion system. The device contains an off-gas line to be connected to the combustion system for receiving the off-gas. A catalytic converter is disposed in the off gas line. A reservoir for holding a reducing agent solution is provided. A metering device is connected to the reservoir and to the off-gas line for metering the reducing agent solution into the off-gas. A regulating system is connected to the metering device and is configured to meter a volume of the reducing agent solution according to demand, taking into account a density of the reducing agent solution.
The regulating system is configured in such a manner as to be suitable for regulating the density of the reducing agent solution. In this context, the term regulating encompasses both passive monitoring or measuring and actively controlling or setting the density of the reducing agent solution.
For regulating in the sense of monitoring, the regulating system advantageously contains a temperature sensor that can be used to measure the temperature of the reducing agent solution.
For regulating in the sense of controlling, the regulating system preferably contains a temperature control device that can be used to set a defined temperature of the reducing agent solution.
In accordance with a concomitant feature of the invention, the regulating system contains a memory element storing characteristic curves defining a relationship between the temperature and the density of the reducing agent solution.
The advantages that have been explained with regard to the method also apply, mutatis mutandis, to the device.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and a device for the catalytic reduction of nitrogen oxides contained in the off-gas from a combustion system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.